Integrated circuit fabrication begins with a thin slice of single crystal semiconductor, usually silicon, and employs a combination of physical and chemical processes to create the integrated circuit structure. The fabrication process is very sensitive to particulate contamination and airborne particulates must be minimimized during the fabrication sequence, as even very small particles on the wafer surface may cause device defects. A reduced particle fabrication ambience is normally achieved by the use of vertical laminar-flow (VLF) clean rooms. Lint-free garments are normally worn by persons working within those clean rooms to minimize operator-borne particles.
Tests have shown, however, that clean rooms can only reduce airborne particulate levels to approximately 10-20 particles per cubic foot in areas where people are working, though clean levels of less then one particle per cubic foot can be achieved in areas aerodynamically isolated from people. Thus, where particle contamination densities of less than one particle per cubic foot are required, the semiconductor slices currently cannot be exposed to VLF air in a clean room occupied by people.
Various techniques have been developed for aerodynamically isolating selected portions of a clean room from people. One such technique involves the use of air-drops or panels dropped from the filter ceiling of the clean room to enclose a people-free area, such as a process machine. Another technique uses a portable carrying apparatus, termed a clean carrier, for storing and transporting semiconductor slice cassettes within the people-occupied area of the clean room. Such a carrier is described in co-pending application Ser. No. 644,282, filed Aug. 27, 1984 entitled SEMICONDUCTOR SLIDE CASSETTE CARRIER, and shields the semiconductor slices from VLF air in the people-occupied environment.
Previously, when it was desired to load a cassette of slices or the slices themselves into a machine, the loading operation was performed by hand. Thus even using a clean carrier in combination with air panels, the slices would nonetheless be exposed to particulate contamination during the loading process. A need has arisen therefore for a mechanism for transporting the semiconductor slices from the clean carrier to a process machine without exposing the slices to the people-occupied environment. The present invention is thus directed to a transport unit designed to be used in conjunction with the clean carrier of the co-pending patent application identified above to transport cassettes and slices between the clean carrier in the people-occupied area of the clean room and a process machine area aerodynamically isolated from people, without exposing the slices. to particulate contamination or introducing particulate contamination to the process machine area.